A magnetic random access memory (MRAM) is expected and actively developed as a nonvolatile memory capable of performing a high-speed operation and rewriting an infinite number of times. In the MRAM, a magnetic material is used as a memory element, and data is stored in the memory element as a magnetization direction of the magnetic material. Some methods for switching the magnetization direction of the magnetic material are proposed. Those methods have in common with usage of a current. To put a MRAM into practical use, it is important to reduce a writing current as much as possible. According to the non-patent literature 1, it is required that a wiring current should be reduced to be equal to or less than 0.5 mA, preferably equal to or less than 0.2 mA.
The most general method of writing data in a MRAM is a method switching a magnetization direction of a magnetic memory element by a magnetic field which is generated by passing a current through a wiring line for a writing operation prepared on the periphery of the magnetic memory element. Since this method uses the magnetization switching caused by the magnetic field, the MRAM can theoretically perform writing at the speed of 1 nano-second or less and thus, this MRAM is suitable for a high-speed MRAM. However, a magnetic field for switching magnetization of a magnetic material securing thermal stability and resistance against external disturbance magnetic field is generally a few dozens of [Oe]. In order to generate such magnetic field, a writing current of about a few mA is needed. In this case, a chip area is necessarily large and power consumed for writing increases. Therefore, this MRAM is not competitive with other kinds of random access memories. In addition, when a size of a memory cell is miniaturized, a writing current further increases and is not scaling, which is not preferable.
Recently, as methods to solving these problems, following two methods are proposed. The first method is a method using a spin transfer magnetization switching. This method uses a laminated layer including a first magnetic layer which has magnetization that can be switched, and a second magnetic layer which is electrically connected to the first magnetic layer and has magnetization that is fixed. In the method, the magnetization in the first magnetic layer is switched by using an interaction between spin-polarized conduction electrons and localized electrons in the first magnetic layer when a current flows between the second magnetic layer and the first magnetic layer. The spin transfer magnetization switching is generated when a current density is equal to or more than a certain value. Accordingly, as the size of the element decreases, the writing current is also reduced. In other words, the spin transfer magnetization switching method is excellent in scaling performance. However, generally, an insulating film is provided between the first magnetic layer and the second magnetic layer and a relatively large current should be made to flow through the insulating film in the writing operation. Thus, there are problems regarding resistance to writing and reliability. In addition, there is concern that a writing error occurs in a reading operation because a current path of the writing operation is the same as that of the reading operation. As mentioned above, although the spin transfer magnetization switching method is excellent in scaling performance, there are some obstacles to put it into practical use.
The other method is a method using a current driven domain wall motion effect. The magnetization switching method using a current driven domain wall motion effect can solve the above-mentioned problems that the spin transfer magnetization switching method is confronted with. For example, MRAMs using the current driven domain wall motion effect are disclosed in the patent literatures 1 to 5. Specifically, the patent literature 3 discloses a magnetoresistive effect element formed of a magnetic material film having a magnetization in a thickness direction. In a MRAM using the current driven domain wall motion effect, generally, magnetization of both end portions of the first magnetic layer having magnetization which can be switched are fixed such that the magnetization of one end portion is approximately anti-parallel to that of the other end portion. In the case of such magnetization arrangement, a domain wall is introduced into the first magnetic layer. Here, as reported in the non-patent literature 2, when a current flows through the domain wall, the domain wall moves in the direction same as the direction of the conduction electrons. Therefore, data writing can be realized by making the current flow inside the first magnetic layer. The current driven domain wall motion is generated when the current density is equal to or more than a certain value. Thus, this MRAM has the scaling property similar to the MRAM using the spin transfer magnetization switching. In addition, in the MRAM element using the current driven domain wall motion, the writing current does not flow through the insulating layer in the magnetic tunnel junction and the current path of the writing operation is different from that of the reading operation. Consequently, the above-mentioned problems caused in the spin transfer magnetization switching can be solved.
However, the inventors have now discovered the following facts.
In a MRAM using the current driven domain wall motion effect, there is concern that the absolute value of the writing current becomes relatively large. Other literatures other than the non-patent literature 2 report observations of the current induced domain wall motion, and the current density of approximately 1×108 A/cm2 is required for the domain wall motion. For example, it is assumed that a width and a thickness of a layer where the domain wall motion arises are 100 nm and 10 nm, respectively. In this case, the writing current is 1 mA. In order to reduce the writing current less than this value, it may be considered that the film thickness should be thinner than before. However, in this case, it is known that the threshold current density required for writing further increase (for example, see the non-patent literature 3).
Furthermore, in order to generate the current driven domain wall motion, a width of a layer where the domain wall motion arises is required to be equal to or less than 10 nm. This leads to great difficulty in fabricating the layer. In addition, there is concern that using the wiring current with the current density of approximately 1×108 A/cm2 for the domain wall motion causes the electromigration and the negative impact due to temperature increase.
In order to solve the above-mentioned problems, the inventors have now considered the following magnetoresistive effect element. The magnetoresistive effect element includes: a magnetization free layer; a spacer layer provided adjacent to the magnetization free layer; a first magnetization fixed layer provided adjacent to the spacer layer and opposite to the magnetization free layer with respect to the spacer layer; and at least two second magnetization fixed layers provided adjacent to the magnetization free layer. The magnetization free layer, the first magnetization fixed layer and the second magnetization fixed layers have magnetization components in a direction perpendicular to the film surface. The magnetization free layer includes: two magnetization fixed portions and a domain wall motion portion arranged between the two magnetization fixed portions. The magnetization of one of the two magnetization fixed portions and the magnetization of the other of the two magnetization fixed portions are fixed approximately anti-parallel to each other in the direction perpendicular to the film surface by the two second magnetization fixed layers. The domain wall motion portion has a magnetic anisotropy in the direction perpendicular to the film surface. The domain wall motion portion of the magnetization free layer, the spacer layer and the first magnetization fixed layer constitutes a magnetic tunneling junction (MTJ).
In this magnetoresistive effect element considered by the inventors, when the data writing operation is performed, the writing current flows through one of the two second magnetization fixed layers, the magnetization free layer, and the other of the two second magnetization fixed layers. In addition, when the data reading operation is performed, the reading current flows through one of the two second magnetization fixed layers, the magnetization free layer, the spacer layer and the first magnetization fixed layer.
In this way, this magnetoresistive element uses a magnetic material film with perpendicular magnetic anisotropy and the domain wall motion effect. Therefore, a magnetoresistive effect element and a MRAM using the same as a memory cell are provided, in which the writing current is sufficiently reduced, the current density thereof is reduced, and the magnetization switching is performed by using the current driven domain wall motion effect.
However, in this magnetoresistive effect element, the magnetization free layer constituting the domain wall motion portion for writing and the magnetization free layer (domain wall motion portion) constituting the magnetic tunneling junction (MTJ) for reading are shared. Therefore, optimization of the magnetic properties of the domain wall motion portion as the writing member and optimization of the magnetic properties of magnetization free layer (the domain wall motion portion) of the MTJ as the reading member cannot be respectively executed independently. As a result, the reduction of the writing current and the increase of the reading magnetoresistive effect (MR) are not compatible.